Nitrous oxide is a gaseous pharmacological agent with a broad range of clinical applications in medicine and dentistry. Despite the use of nitrous oxide as a clinical analgesic agent, its mechanism of action remains somewhat nebulous. Recent research conducted in our laboratory and elsewhere implicates an interaction of subanesthetic concentrations of nitrous oxide with endogenous opioid systems. Based on various experimental findings, we hypothesize that nitrous oxide analgesia might result from drug-stimulated neuronal release of specific endogenous opioid peptides (EOPs) with subsequent activation of central analgesia-mediating opioid receptors. Over the past several years, research efforts in our laboratory have been devoted to further eluci- dation of this opioid connection in nitrous oxide analgesia. The objectives of this proposed research are to: (1) identify the subtype(s) and location(s) of central opioid receptors that mediate nitrous oxide analgesia; (2) quantify and identify the increased EOPs in artificial cerebrospinal fluid perfusate collected from both anesthetized and conscious, centrally perfused rats exposed to nitrous oxide; and (3) correlate the degree of nitrous oxide analgesia with changes in regional brain and spinal cord levels of EOPs. Towards these ends, both biological and chemical approaches will be employed. The biological approaches will include rat stereotaxic surgery and intra- cerebroventricular/intrathecal/intracerebral microinjection technique, and analgesia testing (hot plate test). The chemical approaches will include radioimmunoassay for EOPs in ventricular-cisternal/intrathecal perfusate and nervous tissues of rats !exposed to nitrous oxide, and fractionation by high performance liquid chromatography (HPLC) for EOPs from perfusate and tissue samples from rats exposed to nitrous oxide. Other parameters to be monitored include inspired anesthetic agents, body temperature, blood pressure and blood chemistry. The long term goal of this continuing research is to systematically analyze the opioid nature of the drug actions and drug effects of nitrous oxide. The findings of this research should advance our understanding and knowledge of the pharmacology of nitrous oxide, which appears to be a unique and useful therapeutic and investigative opioid agent. This work should contribute to more efficacious or judicious clinical use of nitrous oxide in medicine and dentistry.